Here's why:

* Sulfur (S) has 6 valence electrons.

* Oxygen (O) has 6 valence electrons.

* In a sulfate ion (SO₄²⁻), sulfur is bonded to four oxygen atoms.

* Each oxygen atom shares two electrons with sulfur, forming a double bond.

* This gives sulfur a total of 12 electrons (6 from its own valence shell and 6 shared with the oxygens).

* However, sulfur can only hold 8 electrons in its outer shell.

* The extra 4 electrons (12 - 8 = 4) create a -2 charge on the sulfate ion.